Filtration under pressure has, inter alia, the advantage that a filtrate can be removed from solids in suspension with deposition of a filter cake upon a filter surface from a suspension of the solid phase in the liquid phase, while the liquid phase is under superatmospheric pressure, thereby avoiding or limiting evaporation of the liquid through all phases of the filtering operation including deposition of the filter cake, washing of the filter cake (if desirable) dislodgment of the filter cake from the filter surface with, possibly, intervening drying of the solids on the filter cake.
To this end filters have been provided within a pressurizable vessel (see Information Chimie, No. 145, June 1975, pp. 302-304.
In these earlier rotary cell filters, the filter cells, extending radially outwardly from the hub or shaft of the rotor, are, as the latter rotates, immersed in the suspension which is received in a trough and consists of the solid and liquid phases, whereupon a pressure differential is applied across the filter material of the cell inducing the filtrate to pass through and resulting in the collection of a filter cake upon the surface. The differential is produced by reducing the pressure in the filter cell below that prevailing in the vessel and thereby effectively evacuating the cell. This positive or inward pressure differential drives the filtrate through the filter material and permits the filtrate to be withdrawn.
At a subsequent point the travel of the filter cells, i.e. of rotation of the rotor, the filter cells are withdrawn from the suspension and the filter cake is removed from the filter material either by a mechanical stripping of by raising the pressure within the filter cells to a level above that prevailing in the pressurizable vessel, thereby applying an outward or negative pressure differential which dislodges the filter cake. The pressure in both cases can be created by compressed air and the reduced pressure can be created by communicating atmospheric or ambient pressure to the filter cells. The negative pressure differential can result in a pressure wave or shock to effect this dislodging of the filter cake.
Prior to removal of the filter cake filter material, the filter cake can be subjected to various treatments on the filter cells, e.g. a washing whereby the filter cake is contacted with another liquid while the interior of the cells at the reduced pressure, and a drying during which this reduced pressure is maintained to free the filter as much as possible from liquid.
The filter cells are provided in a pressure vessel into which the suspension is fed under pressure and above the suspension there is usually provided a pressurized gas space in which the gas (for example air) is maintained under a constant pressure.
The pressure in the vessel increases the filtering effect and permits filtration by pressure differential of liquids which may have a high volatility such that it would flash or evaporate upon the application of a subatmospheric pressure equivalent to the pressure differential.
The conventional rotary pressure filters generally comprise sector-shaped filter cells which are assembled into filter disks lying in planes perpendicular to the axis of the rotor and extending outwardly from the filter shaft with means being provided between these disks for removing the filter cake and treating the filter cake.
As a practical matter it has been found that such filters can be operated with filtering areas of 1.6 to 1.7 m.sup.2 per m.sup.3 of the enclosed volume of the vessel. Naturally as attempts are made to increase the filter area in this type of structure, problems are encountered with removal of the filter cake, the treating devices, etc. and with access of the suspension to the filter surfaces.
While these earlier filters are advantageous for many purposes, their use involves disadvantages which have limited their applicability. For example, the shape of the filter cells, especially when a large number of relatively small filter cells are to be provided, is not compatible with the commercially available shapes of the filter material, e.g. filter fabric, so that cutting and assembling of the filter material in a tedious and time-consuming manner is required. Furthermore, the filter disks cannot be readily mounted on and dismounted from the rotor. It is also a disadvantage that the desired filter surface area can be provided only in stages with significant jumps from one stage to the next, i.e. corresponding to the addition or removal of an entire filter disk.
Of perhaps greater significance is the fact that the filtrate must be led away through the rotor shaft which is of limited flow cross section. Obviously, when attempts are made to increase the flow cross section and hence the diameter of this shaft, the volume of the vessel which might otherwise be occupied by filter surface is taken up with the increased-diameter shaft, thereby reducing the available filter area per unit volume of the vessel.
Experience has shown that with more than four filter disks, two fluid distribution heads are required, one at each end of the filter shaft. Aside from the increased cost which the duplicated distribution head entails, such systems have the disadvantage that it is not always possible to obtain uniform removal of the filtrate and hence the thickness of the cake built up on the filter surfaces is not always constant or the same over the area of each filter disk or from filter disk to filter disk. This means that the filtering effectiveness may vary over the filter surfaces and that at least portions of the recovered solids may have an inordinately high residual moisture content.